In today's mobile communication networks of the third generation called 3G-networks for short, the theoretically possible DL bitrates may reach up to 14.4 Mbit/s in so called HSDPA (High-Speed Download Packet Access) networks and in the planned 3GPP LTE (Third Generation Partnership Project Long Term Evolution) DL bitrates over 300 Mbit/s will be possible. These high bit-rates are among others achieved by adaptive signal modulation and coding techniques, where the optimal ratio between the amount of user data and redundant data is determined in order to achieve the best utilization of the resources available on the radio link.
Especially, link adaptation is used as a fundamental technique in modern mobile communication systems. With link adaptation or transport format selection, the channel coding rate and modulation scheme is chosen based on so-called CQI reports. These reports are derived at the receiver to reflect channel quality and interference levels, and subsequently transmitted over a signaling channel. CQI reports may be issued by a mobile terminal and sent to a base station or a Node B or issued by the base station or Node B and transmitted to the one or more mobile terminals. Using the received CQI reports, a transmitter selects the channel coding rate and modulation scheme to transmit as much user data as possible using as little resources as possible.
Typically, the CQI reports are accurate only for certain mobile speeds and multipath channel types. Here, the term mobile speed refers to the speed of movement of a mobile terminal in a coverage area associated with a base station or Node B. At low mobile terminal speeds, i.e. up to 1 m/s the transmission conditions will be changing slowly and signal fading will be slow as well. Thus, CQI reports sent from the moving mobile terminal to the base station will approximately reflect the real transmission conditions on the radio channel.
In other situations the CQI reports are inaccurate to some extent. For example, for a mobile terminal moving at higher speeds, such as at 10 m/s or faster, the channel conditions will be changing fast due to fast signal fading. Thus the CQI-reports transmitted to a base station at one instant will most probably not reflect the real channel conditions once they are received at the base station. This means that the link adaptation also becomes inaccurate, causing the quality predicted at the transport format selection to differ from the quality at the time of the reception. One way of compensating for such inaccuracies that has been devised in known technology is a function called CQI adjustment. This function effectively constitutes an outer loop for the link adaptation. With CQI adjustment, the received quality is compared to a quality target. In case the reported signal quality differs from the quality target, the reported CQI is adjusted by a corresponding value.
However, in existing solutions, the CQI adjustment relies heavily on that the transport formats and transmit power are chosen to obtain a certain target BLER. In case other quality targets are desirable, the BLER target method may not lead to an optimum solution and possibly cause unpredictable behavior for the transmission and reception of data. Deviations from the BLER target will in this case be caused not only by inaccurate CQI reports but more commonly by selection of transport format.